This invention relates to mixing apparatus for intermixing two or more reactants and, more particularly, to mixing apparatus for manufacturing photographic emulsions.
Most state-of-the-art manufacturing processes used in the photographic industry expose emulsion grains to local high concentrations of silver nitrate in the vicinity of the silver nitrate reagent introduction pipes. (See for example U.S. Pat. Nos. 3,415,650, 3,692,283, 4,289,733, 4,666,669, 5,096,690, 5,238,805). Such exposure is undesirable because of the uncontrolled amount of reduced silver centers that may be created on the grain surface. The variable amount of reduced silver centers lead to variability in photographic sensitivity of the emulsions, which is undesirable.
In processes where an additional mixing vessel is used to mix silver nitrate with alkali halide prior to introducing them as fine silver halide grains into the reaction vessel (see for example U.S. Pat. Nos. 5,145,768 and 5,334,359), such an exposure is avoided but, additional complexities are created related to the additional mixing vessel and transport of material from that additional mixing vessel to the reaction vessel. When the mixing vessel is separate from and positioned externally to the main reaction vessel, problems arise due to the growth of fine grains during the solution delivery from the mixing vessel to the main reaction vessel. Such a process usually does not meet the requirement of grain formation in a time period as short as realized in the conventional method. When the mixing vessel is immersed in the reaction vessel, the above problem is apparently solved only when a separate heavy duty mechanical stirrer is provided near the discharge slit of the mixing vessel for immediate uniform mixing of the discharged solution with the reaction mixture. It is, however, well known that a well stirred mixing vessel has an exponential distribution of residence times (cf. O. Levenspiel, Chemical Reaction Engineering, 2nd Edition, Chapter 9). Therefore, a small fraction of discharge fluid bypasses the mixing process inside the mixing vessel and microscopic pockets of high concentration silver nitrate solution are expected to be discharged into the reaction vessel. Furthermore, when the discharge fluid meets the reaction mixture in the space between the two heavy duty mechanical stirrers, the mixing intensity is lower than that near the stirrer blades, so the pockets of high concentration silver nitrate solution are not immediately eliminated. Also, the discharge slit of the mixing vessel has to be provided with a back flow preventing valve to prevent reaction mixture from flowing into the mixing vessel, providing yet another operational complexity in a manufacturing environment.
U.S. Pat. No. 5,690,428 to Bryan et al. teaches a mixing device that includes concentric tubes for supplying solutions to a mixing rotor. The mixing rotor in combination with the supply tubes creates a non-planar, annular reaction zone that includes step changes in diameter thereof, and therefore, multiple turns in the flow path through the reaction zone.
There is continuing need for manufacturing high sensitivity photographic emulsions with tightly controlled sensitivities. Since prior art mixing apparatus subject emulsion grains to variable high concentrations of silver nitrate in the reaction vessel, tight control of sensitivity of the emulsion being manufactured is difficult. In prior art processes where the emulsion grains are not exposed to high concentrations of silver nitrate, the problem is either that of longer grain formation time than the conventional process, or that of increased operational complexity of the manufacturing process, resulting from the placement of at least two separate heavy duty mechanical stirrers in close proximity of each other.
It is therefore an object of the present invention to provide an apparatus for mixing two or more reactants which provides a very short residence time in the apparatus.
It is a further object of the present invention to provide an apparatus for mixing two or more reactants which obviates any back flow in the apparatus.
Yet another object of the present invention is to provide an apparatus for mixing two or more reactants which prevents the formation of short-circuiting flow paths therethrough.
Still another object of the present invention is to provide an apparatus for mixing two or more reactants which prevents the formation of dead flow pockets therein.
Briefly stated, the foregoing and numerous other features, objects and advantages of the present invention will become readily apparent upon a review of the detailed description, claims and drawings set forth herein. These features, objects and advantages are accomplished by forming a mixing apparatus that includes a generally planar reaction zone inside the main reaction vessel such that the reaction mixture contained in the main reaction vessel never backflows into the planar reaction zone. The invention accomplishes efficient mixing inside the reaction zone, as well as efficient mixing of the reaction products produced in the reaction zone with the reaction mixture in the main reaction vessel. Within the planar reaction zone, the two reactants are mixed and reacted. The reaction products exit the reaction zone directly into the reaction mixture contained in the main reaction vessel. There is no connecting or intermediate flow path. In other words, there is a direct interface between the reaction zone and the reaction mixture contained in the main reaction vessel. In the production of photographic emulsions, silver nitrate and alkali halide solutions are mixed and reacted such that they are converted into the fine silver halide grains by the time they leave the generally planar reaction zone and mix with the reaction mixture. The generally planar reaction zone includes a rotating disc which defines one surface or boundary of the generally planar reaction zone. The two reactants are directed in separate and concentric annular flow paths at the substantially planar surface of a rotating disc. In the production of photographic emulsions, silver nitrate and alkali halide solutions are directed in separate and concentric annular flow paths at the planar surface of a rotating disc. The rotating disc aids in the mixing of the silver nitrate and alkali halide solutions. Further, the rotating disc may act, at least partially, as a pump impeller accelerating the reacting silver nitrate and alkali halide solutions toward the perimeter of the rotating disc in a generally radial or spiral flow path through the generally planar reaction zone. The rotating disc should provide enough pumping to at least overcome head losses resulting from the flow of the liquid through the reaction zone. This, in combination with the flow rates and pressures of the two reactants, and the generally planar reaction zone, ensures that there is no back flow of the silver nitrate and alkali halide solutions in the generally planar reaction zone. In other words, the rotating disc, the flow rates and pressures of the two reactants, and the geometry of the reaction zone obviate the formation of stagnant pockets in the planar reaction zone. As the reacted silver nitrate and alkali halide solutions exit the reaction zone, they immediately mix with the reaction mixture in the main reaction vessel. Through the control of the pressure and flow rates of the silver nitrate and alkali halide solutions into and through the reaction zone, and disc rotation, backward mixing of the reaction mixture from the main reaction vessel into the planar reaction zone is prevented. The residence time of the fluid in the reaction chamber is so small that the fine grains that are generated are ejected into the main reaction vessel very quickly after the formation thereof. Thus the present invention avoids exposure of emulsion grains in the main reaction vessel to high concentrations of silver nitrate without introducing complexities of the prior art where two stirrers have to be placed in close proximity. In general, the planar reaction zone is advantageous because it produces a more uniform distribution of fine grains, which can be used to produce a narrower distribution of silver halide emulsion grains. The uniformity of flow field also improves the scalability of the precipitation process. Thus, the present invention solves the problems encountered with the prior art processes and apparatus and enables manufacturing of high sensitivity photographic emulsions with tightly controlled sensitivities.
The present invention is being described herein with specific relationship to the mixing into the main reaction vessel of reacted silver nitrate and alkali halide solutions to form radiation-sensitive emulsions, specifically, silver halide emulsion grains. However, those skilled in the art will recognize that this invention may be applicable to any precipitation process that produces particles of sparingly soluble materials. For example, the apparatus of the present invention may be used for gold or silver chalcogenides.